1. Field of the Invention
The present invention relates to a photoelectric conversion device having a charge storage region whose potential can be controlled by means of a capacitor.
2. Related Background Art
FIG. 1A is a schematic plan view showing a photoelectric conversion device described in the Official Gazette of an EPC laid-open publication No. 0132076, and FIG. 1B is a cross section along line I--I of FIG. 1A.
In the figures, photosensor cells are formed disposed on an n.sup.+ silicon substrate 101, each photosensor cell being electrically insulated from adjacent cells by an element isolation region 102 made of, such as SiO.sub.2, Si.sub.3 N.sub.4, or polysilicon.
Each photosensor cell is constructed as in the following:
On an n.sup.- region 103 of low impurity density which is formed by epitaxial technique or the like, there is formed a p region 104 by doping p-type impurities within which p region an n.sup.+ region 105 is formed by impurity diffusion technique or ion implantation technique. The p region 104 and n.sup.+ region 105 constitute the base and emitter of a bipolar transistor, respectively.
On the p region 104 and n.sup.+ region 105 formed on the n.sup.- region 103, there is formed an oxidized film 106 on which a capacitor electrode 107 having a predetermined area is formed. The capacitor electrode 107 faces the p region 104, the oxidized film 106 being interposed therebetween. Thus, by applying a voltage pulse to the capacitor electrode 107, the potential of the p region 104 at a floating state can be controlled.
In addition, there are formed an emitter electrode 108 connected to the n.sup.+ region 105, an interconnection 109 for reading a signal from the emitter electrode and outputtting it to an external circuit, an interconnection 110 connected to the capacitor electrode 107, an n.sup.+ region 111 of high impurity density on the back of the substrate 101, and an electrode 112 for applying a potential to the collector of the bipolar transistor.
Next, the fundamental operation of the above-described photosensor will be described. Assume that the p region 104 or base of the bipolar transistor is at an initial state biased to a negative potential. Light 113 is applied to the p region 104 or the base of the the bipolar transistor to store the charge corresponding to light quantity in the p region 104 (storage operation). The base potential varies with the stored charge so that current flowing through the emitter and collector is controlled. By reading a potential change from the emitter electrode at a floating state, an electrical signal corresponding to the incident light quantity can be obtained (readout operation). To eliminate the stored charge in the p region 104, the emitter electrode 108 is grounded and the capacitor electrode 107 is applied with a positive pulse voltage. Since the p region 104 is forward-biased relative to the n.sup.+ region 105 by applying the positive pulse voltage, the stored charge is eliminated. As the positive pulse voltage or refreshing pulse falls, the p region 104 resumes the initial state biased to a negative potential (refreshing operation). The above operations, i.e., storage, readout and refreshment are repeated.
In summary, the charge generated by incident light is stored in the p region 104 or base so that current flowing through the emitter electrode 108 and collector electrode 112 can be controlled. Thus, the stored charge is amplified at each cell and thereafter is read so that a high output and sensitivity as well as a low noise can be achieved.
The base potential V.sub.p generated by the holes stored in the base through light excitation is given by Q/C, wherein Q represents the charge quantity of holes stored in the base, and C represents a capacitance connected to the base. As apparent from the above relationship, a high integration of the photosensor leads to a reduction in cell size as well as a reduction in values of Q and C, so that the potential V.sub.p generated through light excitation is maintained substantially constant. Consequently, this method is considered also effective for future high resolution photosensors.
However, it is necessary to dispose narrow and elongated photosensor arrays in a photoelectric conversion device if high integration and resolution are intended without lowering an output voltage. In this case, resistance in the longitudinal direction of the array becomes large so that a potential distribution is generated in the active region of a photosensor cell, resulting in a reduction in response speed and the occurrence of an afterimage phenomenon.